1. Technical Field
The present disclosure is related to plasma applicators and, in particular, to a compact microwave plasma applicator which utilizes conjoining electric fields.
2. Discussion of Related Art
In semiconductor processing, plasma generators are often employed upstream of a processing chamber. In plasma generators, energy is typically coupled to a gas flowing through a plasma discharge tube which is located in a microwave cavity, and plasma is excited in the gas by the microwave energy. Plasma products flow downstream through the plasma discharge tube, into the process chamber, and impinge on a workpiece such as a semiconductor wafer.
In plasma generation systems, energy efficiency and cost are important considerations. In conventional high-energy, e.g., greater than 5 kW microwave power, plasma generators, the plasma discharge tube can reach extremely high temperatures. To reduce the effects of the high temperatures, cooling systems are typically employed. Generally, liquid coolants, e.g., water, are circulated through a copper winding/coil around the tube to remove excess heat. These liquid cooling systems tend to be very expensive, but are unavoidable because of the high temperatures at the high power levels. It is preferable to use air cooling in plasma generators because of its reduced complexity and cost, but air cooling is typically only useful in systems using lower power, e.g., 1 kW or less microwave power, because air cooling is effective against the relatively lower temperatures generated in the plasma discharge tube.
Also, because of non-uniform excitation energy patterns, e.g., electric fields, used to excite the gases to generate plasma in the plasma discharge tube, plasma can be generated in a non-uniform spatial pattern, resulting in “hot spots” at which the plasma energy and, therefore, temperature, in the tube is especially high. These “hot spots” induce inefficiency in the plasma generation process. Also, they present a non-uniform temperature control burden to the cooling system, whether air cooling or water cooling is used. For this and other reasons, it is highly desirable that the microwave energy, and, in particular, the electric fields, used to generate plasma in the plasma discharge tube be spatially uniform, so that the resulting plasma is relatively free of hot spots.